1. Field of the Invention
The present invention relates to a measurement method of resonant nonlinearity in a nonlinear optical fiber using a long period fiber grating pair. In more detail, it relates to a characteristic evaluation method that can simply, but accurately, measure the nonlinear optical characteristics of an optical fiber.
2. Description of the Related Art
Recently in the field of optical communication, researches on all-optical switch element using resonant nonlinearity of nonlinear optical fiber is being actively in progress. Here, xe2x80x9cresonant nonlinearityxe2x80x9d means for nonlinearity obtained in an optical communication band by using a pump light source matching with the light absorption band of nonlinear material doped into an optical fiber. As a foundation of these researches, measurement of nonlinearity of the optical fiber should be performed.
In the prior art, the following methods has been used for measuring resonant nonlinearity of a nonlinear optical fiber:
One is a method using xe2x80x9cpulse broadeningxe2x80x9d. This method is based on a characteristic that, when a pulse having a very narrow frequency width transmits through a nonlinear optical fiber, the pulse width is changed by resonant nonlinearity of the optical fiber. Since the pulse width is generally broadened by nonlinearity of an optical fiber, the method is called by xe2x80x9cpulse broadeningxe2x80x9d, and nonlinearity of an optical fiber can be measured by the extent of pulse broadening.
Another method is a pump-probe method. This method employs a laser, which is able to scan a wide frequency band, as a light source and measures nonlinearity of an optical fiber by observing light absorption change in a wide frequency band according to the output power of a pump light source. This method is not only used for measuring nonlinearity of an optical fiber but also widely used for measuring nonlinearity of general materials.
Besides, there is another method using a Mach-Zehnder interferometer. This method splits a light source 50 to 50, transmits one beam through an optical fiber having resonant nonlinearity and the other beam through an optical fiber not having resonant nonlinearity and a phase modulator, and combines the two beams thereafter. Then, there exists a phase difference between the two beams, and the method compensates the difference by using a phase modulator to measure the nonlinearity.
These are frequently used methods for measuring nonlinearity of an optical fiber, however, they have the following disadvantages:
First, for using a pulse broadening method, the key element is a pulse having a very narrow frequency width, however, it is not easy to produce a pulse having such a narrow frequency width. And a pump-probe method requires an expensive laser light source that is able to scan a wide frequency band.
And for applying a method using a Mach-Zehnder interferometer, since the method basically uses interference between two beams, it is required to arrange optical elements installed on their optical paths carefully to cause the interference. Additionally, it requires a comparably expensive phase modulator.
The present invention is proposed to solve the problems of the prior art mentioned above. It is therefore the object of the present invention to provide a cheap and easy measurement method of resonant nonlinearity in a nonlinear optical fiber that dissolves the disadvantages of prior art.
To achieve the object mentioned above, the present to invention presents a measurement method of resonant nonlinearity in a nonlinear optical fiber using a long period fiber grating pair characterized by: connecting a nonlinear optical fiber (20) between a first long period fiber grating (10a) and a second long period fiber grating (10b); injecting light from a pump light source (30) for causing resonant nonlinearity, and at the same time, injecting light from a light source (40) of which the nonlinearity is to be measured through the first long period fiber grating (10a) or the second long period fiber grating (10b) at one side; transmitting the light from the two light sources (30,40) through the nonlinear optical fiber (20) and the second long period fiber grating (10b) or the first long period fiber grating (10a) at the other side; and thereafter measuring the light from the light source (40) of which the nonlinearity is to be measured by a spectrum analyzer (60) using the change of interference pattern formed by the long period fiber grating pair (10a, 10b).